Glaucoma is a major cause of blindness worldwide. The blindness that results from glaucoma can involve both central and peripheral vision and can have a major impact on an individual's ability to lead an independent and productive life. Pathophysiologically, glaucoma is an optic neuropathy (a disorder of the optic nerve) observed most typically in the setting of an elevated intraocular pressure. Dramatic and/or prolonged increases in intraocular pressure cause changes in the appearance (“cupping” or “excavation”) and function (“scotomas” or “blind spots” in the visual field) of the optic nerve. If the pressure remains high enough for a long enough period of time, total, irreversible vision loss occurs. High intraocular pressure results from an imbalance in intraocular fluid production versus outflow. Glaucoma surgeries, also referred to as filtering procedures, are designed to improve intraocular fluid balance by augmenting fluid outflow, thereby lowering intraocular pressure.
Many techniques are available for treating an eye with an elevated intraocular pressure. These include various surgical techniques for facilitating flow of fluid from the inside of the eye to the subconjunctival space. The most commonly performed procedure, known as a trabeculectomy, involves creating a surgical fistula between the anterior chamber of the eye and the subconjunctival space. Less commonly, a surgical drainage device may facilitate flow of fluid between the anterior chamber and the subconjunctival space. Some such drainage devices, such as the Molteno, Ahmed, Kruppin, and Baerveldt valves, connect via a shunt tube to an externally fixated, subconjunctival reservoir. Modifications of these devices are available whereby the external reservoir and tube may facilitate flow from the posterior segment of the eye at the pars plana following removal of the vitreous to prevent clogging.